What Causes Earthquakes?

Most Quakes Occur along Faults (Fractures in Earth's Crust)

Elastic Rebound Theory

Here we have a landscape with a road, a fence, and a line of
trees crossing a fault. As the crust moves, the rocks adjacent to the
fault are deformed out of shape (in reality the deformation is spread
across many kilometers - if it were this obvious, earthquake prediction
would be easy).

Eventually the rocks are so stretched out of shape that they cannot
bear the stress any longer. The fault slips, and the stage is set for the
next cycle of strain buildup and release.

Epicenter and Focus

Focus

Location within the earth where fault rupture
actually
occurs

Epicenter

Location on the surface above the focus

Types of Faults

Faults Are Classified on the Basis of the Kind of Motion That Occurs on Them

Joints - No Movement

Strike-Slip - Horizontal Motion (Wrench Faults)

Left-Lateral

Right-Lateral

(San Andreas - 21 Ft. in 1906)

Dip-Slip - Vertical Motion

Normal (Extension)

Reverse or Thrust (Compression)

(San Fernando, 1971

Alaska, 1964 - up to 150 Ft.)

Major Hazards of Earthquakes

Building Collapse

Landslides

Fire

Tsunamis (Not Tidal Waves!)

Safest & Most Dangerous Buildings

Small, Wood-frame House - Safest

Steel-Frame

Reinforced Concrete

Unreinforced Masonry

Adobe - Most Dangerous

Tsunamis

Caused Probably by Submarine Landslides

Travel about 400 M.p.h.

Pass Unnoticed at Sea Cause Damage on Shore

Warning Network Around Pacific Can Forecast Arrival

Whether or Not Damage Occurs Depends on

Direction of Travel

Harbor Shape

Bottom

Tide & Weather

Seismology

Ideally, we'd like to be able to hover above the earth during and earthquake
and watch the earth move beneath us. Since my anti-gravity belt is in the shop
for repairs, the closest we can come is with a pendulum.

Contrary to intuition, an earthquake does not make the
pendulum swing. Instead, the pendulum remains fixed as the ground moves
beneath it.

A pendulum with a short period (left) moves along with the support and
registers no motion. A pendulum with a long period (right) tends to remain
in place while the support moves.

The boundary between the two types of behavior is the natural period of
the pendulum. Only motions faster than the natural period will be
detected; any motion slower will not.

Since earthquake vibrations can have periods of many seconds, we need a
pendulum with a very long period. We can construct a pendulum with a very long
arm, or we can build a compact instrument by building a horizontal pendulum. If
the pendulum is built like a swinging gate, the restoring force (force pulling
it back toward the center of its swing) can be made very weak, and the pendulum
can have a period as long as we like.

Seismic Waves

Seismic waves come in several types as shown below:

P-Waves

Primary (they arrive first), Pressure, or Push-Pull. Material expands and
contracts in volume and particles move back and forth in the path of the
wave. P-waves are essentially sound waves and travel through solids, liquids
or gases. Ships at sea off the California coast in 1906 felt the earthquake
when the P-wave traveled through the water and struck the ship (generally
the crews thought they had struck a sandbar).

S-Waves

Secondary (arrive later), Shear, or Side-to-side. Material does not change
volume but shears out of shape and snaps back. Particle motion is at right
angles to the path of the wave. Since the material has to be able to
"remember" its shape, S-waves travel only through solids.

Surface Waves

Several types, travel along the earth's surface or on layer boundaries in
the earth. The slowest waves but the ones that do the damage in large
earthquakes.

Magnitude and Intensity

Intensity

How Strong Earthquake Feels to Observer

Depends On:

Distance to Quake

Geology

Type of Building

Observer!

Varies from Place to Place

Mercalli Scale- 1 to 12

We can plot earthquake intensity by gathering reports from observers.
Although the reports will be subjective, and vary somewhat, most observers will
agree on the intensity criteria, for example, feeling the quake while driving.
For very strong quakes, damage provides fairly objective measures of intensity.

Isoseismals from the 1906 San Francisco Earthquake

Overall, the pattern is pretty simple: high intensity close
to the San Andreas Fault, dropping off with distance. But why is there a
disconnected island of high intensity in central California?

The band of low (IV) intensity parallel to the coast coincides with the
Coast Ranges. Soils here are very shallow - usually less than a meter to
bedrock. Observers here felt mostly a sharp jolt.

In contrast, the high intensity in central California coincides with
the Central Valley, where young and unconsolidated sediments are
kilometers deep. Unconsolidated material shakes like jelly in an
earthquake.

Note how intensity VI follows the shoreline of San Francisco Bay, where
there are also thick unconsolidated sediments.

Intensity and Geology in San Francisco

At left above is a map of seismic intensity for the 1906 San Francisco
earthquake. At right is a map of depth to bedrock. The pattern
is clear: the greater the depth to bedrock, the stronger the shaking. Candlestick Park, where game 3 of the 1989 World Series
was about to begin, owes its reputation for being a windy ball park to being
near a steep hill. Its location on bedrock meant that fans felt a sharp jolt,
there were a few cracks in the concrete, and little else. (The First Amendment
gives San Francisco the right to call it 3-Com Park if they like - it also gives
me the right to ignore them.) The Marina District was shaken badly because it's
on artificial fill, in fact, much of it is rubble from the 1906 earthquake. The
deep filled valley in northeastern San Francisco is occupied by the commercial
center of the city but the modern construction is steel-frame and was undamaged
in the 1989 earthquake.

San Francisco and New Madrid Compared

The map at left compares the isoseismals from the 1906 San
Francisco earthquake and the 1811-1812 New Madrid quakes.

There is a lot less intensity data for the New Madrid events so local
details are missing. Intensity estimates are based on reports from places
shown as blue dots.

Although the New Madrid events were big, they owe their vast felt areas to
the layer-cake geology of the Midwest. The flat strata and relative lack of
geologic complexity (especially compared to California) mean that seismic waves
travel very efficiently for long distances with little loss of energy.

Magnitude - Determined from Seismic Records

Richter Scale:

Related to Energy Release

Exponential

Magnitude-Energy Relation

4 - 1

5 - 30

6 - 900

1 Megaton = about 7.0

7 - 27,000

8 - 810,000 (About equal to U.S. Energy use for one day)

No Upper or Lower Bounds

3 - One ton of explosives or the World Trade Center collapse

2 - Two loaded semis colliding at 60 mph

1 - A kilogram of explosives, toppling a 50-m tree, or two cars colliding
at 60 mph.

Seismic Risk Level Maps for the U.S.

Seismic Gaps

Areas that haven't had earthquakes in a long time are prime candidates
for the next one.

Are Earthquakes Getting More Frequent?

It was only in 1885 that a seismograph in Europe detected an earthquake in
Japan, and we have global coverage, even for very large events, only since 1900
or so. Below is a graph, based on USGS data, for the annual number of M=7.5 and
M=8 earthquakes from 1900 to 2001.

The high levels between 1900 and 1918 were real. The instruments might have
overrated some events, but also it is still possible that some events were
missed in those years.

There was a steady decline between 1968 and 1984. Curiously, not a single
person during those years asked me whether earthquakes were becoming less
frequent.

Seismology and Earth's Interior

Successive Approximation in Action

Assume the Earth is uniform. We know it isn't, but it's a useful place to start. It's a
simple matter to predict when a seismic signal will travel any given distance.

Actual seismic signals don't match the predictions

If we match the arrival times of nearby signals, distant signals arrive too soon

If we match the arrival times of distant signals, nearby signals arrive too late.